The subject matter disclosed herein relates to heating, ventilation and air conditioning (HVAC) systems. More specifically, the subject matter disclosed herein relates to HVAC systems with falling film evaporators utilizing low or medium pressure refrigerants.
HVAC systems, such as chillers, use an evaporator to facilitate a thermal energy exchange between a refrigerant in the evaporator and a medium flowing in a number of evaporator tubes positioned in the evaporator. In systems with flooded evaporators, the tubes are submerged in a pool of refrigerant. In flooded evaporator systems, the evaporator and condenser are located substantially side-by-side. In a single stage system, liquid refrigerant leaving the condenser will go through a metering device, such as an expansion valve, and a two phase mixture of liquid and vapor refrigerant enters the evaporator from the bottom of the evaporator. In a two stage system including an economizer, after passing through the metering device the liquid and vapor refrigerant mixture flows through the economizer where the liquid refrigerant is metered again, with a second liquid and vapor refrigerant mixture flowing into the bottom of the evaporator.
In a falling film evaporator system, the liquid refrigerant is fed in through the top of the evaporator and falls over the tubes, where it is evaporated. In a stacked arrangement of a falling film system, the condenser is installed on top of the economizer, which is installed on top of the evaporator. In this system, the flow through the components is driven by gravity. If the condenser and evaporator are arranged side-by-side, however, with an evaporator inlet physically higher than the exit of the metering device downstream of the condenser or economizer, the two-phase refrigerant mixture will have to be routed through a two-phase riser into the evaporator.
Traditionally, when using either medium pressure or high pressure refrigerants, the vertical pipe of the riser is sized such that for all flow conditions (lift and flow rate) the mixture's momentum is great enough to ensure constant flow rate into the evaporator. This sizing results in very large frictional pressure drops at large flow rates. This is not an issue with the high pressure refrigerants, however, since the pressure differential due to lift in these refrigerants can accommodate the frictional pressure drops. When using low pressure refrigerants in falling film applications, however, the pressure differential due to lift is about 25% of that of a typical medium pressure refrigerant, severely limiting the frictional pressure allowed while still maintaining control of flow through the system using the metering device.